The two layouts of a two-layer circuit board located on opposite sides of a flat substrate are normally electrically interconnected by passages plated-through the substrate. These passages are usually mechanically drilled and are then e.g., plated-through using an electroplating process. The layouts of the conductor planes are produced in process steps largely independent of the production of the plated-through holes, conventionally using photochemical and wet chemical processes and flat extensions of the conducting paths or so-called lands are provided around the openings of the holes on both sides. The lands associated with a plated-through hole must necessarily precisely be opposite one another and must at least be sufficiently large that they still extend around the opening of the plated-through hole when summating all the imprecisions which are possible in the method.
The upper limit for the functional density of such circuit boards is inter alia determined by the diameter of the drilled passages, which is minimally at least 0.2 mm and by the dimensions of the associated lands. This upper limit, as obtained with the method using drilled holes, has long been much too low for the requirements made on circuit boards and is therefore circumvented by super-imposing a large number of circuit board planes. The disadvantages inherent in such a development are known and have led to the wish for increasing the functional density within the conductor planes, so that the necessary number of conductor planes can again be limited.
Foil technology offers a number of possibilities, the circuit boards being produced from metal-clad foil material and, instead of the passages for the plated-through holes being drilled, they are etched and in particular plasma etched. The starting product is normally a foil or film of an electrically insulating polymer, e.g. polyimide or epoxy resin coated on both sides with metal, e.g. copper.
For producing the plated-through holes the metal coatings are removed photochemically on both sides at the locations of the openings of the passages. The polymer layer is then etched through at the exposed points from both sides to form a through hole and this typically takes place by plasma etching. These passages are e.g. plated-through by electroplating. Then in further, normally photochemical process steps, the layouts are structured on the metal coatings.
Plated-through holes produced according to such a method can have diameters down to approximately 5/100 mm, so that the functional density is drastically increased compared with the drilling method referred to hereinbefore. Great importance is attached in this method to the dimensioning and positioning of the lands, because it is not only the plated-through holes per se, but also the lands which take up space in the conductor planes and consequently determine the functional density.
However, as the sizes for conductor structures produced according to foil technology are approximately a power of 10 smaller as compared with the sizes obtainable by drilling technology, as a result of the plated-through holes and their tolerances, layout requirements become more important, particularly as the tolerances retain the same order of magnitude. It would therefore be desirable for an optimum utilization of the advantages of foil technology to reduce or obviate such layout requirements.